Move a bunch of stuff out of k5-int.h

Move internal declarations from k5-int.h to more localized headers
(like int-proto.h) where appropriate.  Rename many symbols whose
prototypes were moved to use the k5_ prefix instead of krb5int_.
Remove some unused declarations or move them to the single source file
they were needed in.  Remove krb5_creds_compare since it isn't used
any more.
make depend